Heretofore, ethylenic copolymers such as ethylene-α-olefin copolymers have been being used widely in various fields as popular resins. Ethylenic copolymers are produced in the presence of heterogeneous catalysts. However, ethylenic copolymers as produced in the presence of heterogeneous catalysts have a broad molecular weight distribution and a broad branching degree distribution, and contain a large amount of low-molecular components having a high branching degree. Therefore, it is known that films to be formed from such ethylenic copolymers have low impact strength, poor heat-sealing properties and low ESCR. On the other hand, as having a low degree of anisotropy (imbalance between the lengthwise strength and the widthwise strength), the films are advantageous in that they have high tear strength which is important as one physical property of films.
Recently, homogeneous metallocene catalysts having a high catalytic activity and capable of catalyzing high copolymerization have been developed. Ethylenic copolymers as produced in the presence of such metallocene catalysts shall have a narrow molecular weight distribution and a narrow branching degree distribution (that is, the distribution of the number of branches constituting the copolymer is narrow), and contain a small amount of low-molecular components having a high branching degree. Therefore, it is known that films of such ethylenic copolymers can have improved heat-sealing properties, impact strength and ESCR.
Some ethylenic copolymers as produced in the presence of such metallocene catalysts (often referred to as single-site catalysts) are known. For example, U.S. Pat. No. 5,272,236 discloses ethylene-α-olefin copolymers as produced in the presence of metallocene catalysts. Since the copolymers disclosed have a narrow compositional distribution, films of the copolymers may have improved impact strength and ESCR. However, since the films have a large degree of anisotropy, they are defective in that they have low tear strength. On the other hand, single-site, linear low-density polyethylenes are commercially available. For example, known is a commercial product of Exact which is a trade name of Exxon. However, since this has a narrow compositional distribution and comprises a comonomer of hexene-1 or an α-olefin lower than hexene-1, the tear strength of films to be formed from it is not improved to such a degree as expected, though the heat-sealing property of the films may be desirably improved.
Given this situation, various attempts have heretofore been made to blend different copolymers thereby to control the branching degree distribution and the molecular weight distribution of the resulting copolymer mixtures. However, regarding the tear strength and the impact strength of films to be formed from such copolymer mixtures, if one of the two is increased, the other is lowered. At present, therefore, films having both high tear strength and high impact strength are unknown.
Japanese Patent Application PCT-through Laid-Open No. 502710/1991 discloses a composition comprising an ethylenic copolymer as obtained in the presence of a homogeneous catalyst. However, this refers to nothing about the impact strength of films of the composition, though referring to the improvement in the tear strength of the films. Japanese Patent Application Laid-Open Nos. 136195/1994, 136196/1994 and 207057/1994 disclose compositions comprising two different copolymers both having a narrow compositional distribution and characterized by their melt tension but each having a different density and a different melt flow rate (MFR), and the compositions disclosed are said to form films with improved impact strength. However, the compositions disclosed are characterized by comprising copolymers each having a different molecular weight, and are therefore defective in that their molecular weight distribution is broad and that the mechanical strength, such as tensile strength, of the films of the compositions is low.
Accordingly, it is desired to provide an ethylenic copolymer which has a narrow molecular weight distribution and a narrow branching degree distribution, of which the compositional distribution profile can be freely controlled, and which can be formed into films having both high tensile strength and high impact strength irrespective of the molecular weight distribution of the copolymer, and also to provide a composition comprising such an ethylenic copolymer.